Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

One embodiment of the invention includes an electrochemical cell including
a proton exchange membrane and a method of treating nanoparticles using
the same.

Claims:

1. A method comprising:supporting electrically conductive nanoparticles to
be electrochemically treated over a working electrode, immersing the
working electrode with the supported nanoparticles in a liquid
electrolyte solution, immersing a counter electrode in the electrolyte
solution, and immersing a polymer electrolyte membrane in the electrolyte
solution between the working electrode with nanoparticles supported
thereon and the counter electrode to define a working electrode
compartment and a counter electrode compartment of the cell,applying a
potential or a current across the electrodes to treat the nanoparticles.

2. A method as set forth in claim 1 further comprising immersing a
reference electrode on the working electrode side.

3. A method as set forth in claim 2 further comprising providing a gas
purge tube in the liquid electrolyte on the working electrode side of the
cell.

4. A method as set forth in claim 3 further comprising providing a
container for holding the electrolyte solution and a cover over the
container.

5. A method as set forth in claim 1 wherein the working electrode
comprises a first carbon cloth and a gauze comprising a metal supporting
the carbon cloth, and wherein the particles are spread on the first
carbon cloth.

6. A method as set forth in claim 5 wherein the gauze comprises platinum
or gold or graphite.

7. A method as set forth in claim 5 wherein the counter electrode
comprises a second carbon cloth and a gauze material comprising a metal
supporting the carbon cloth.

8. A method as set forth as claim 5 wherein the counter electrode
comprises a second carbon cloth supported by a gauze comprising platinum
or supported Pt nanoparticles.

9. An electrochemical cell comprising:a container and a liquid electrolyte
received in the container;a working electrode, and nanoparticles
supported by the working electrode;a counter electrode; anda polymer
electrolyte membrane separating liquid electrolyte on the counter
electrode side from liquid electrolyte on the working electrode side of
the cell.

10. An electrochemical cell as set forth in claim 9 wherein the
nanoparticles comprise at least one of Pt, Pt alloy, Ni, or other noble
metals or metal alloys.

11. An electrochemical cell as set forth in claim 9 wherein the working
electrode comprises a first carbon cloth supporting the nanoparticles,
and a gauze comprising a metal supporting the carbon cloth.

12. An electrochemical cell as set forth in claim 11 wherein the gauze
comprises at least one of platinum or gold or graphite.

13. An electrochemical cell as set forth in claim 9 wherein the counter
electrode comprises a second carbon cloth supported by a gauze comprising
a metal.

14. An electrochemical cell as set forth in claim 13 wherein the gauze
comprises at least one of platinum or gold or graphite.

15. An electrochemical cell as set forth in claim 9 further comprising a
reference electrode immersed in the liquid electrolyte on the working
electrode side of the cell.

16. An electrochemical cell as set forth in claim 9 further comprising a
gas purge tube immersed in the liquid electrolyte on the working
electrode side of the cell.

17. An electrochemical cell as set forth in claim 16 further comprising a
cover over the container.

18. A method as set forth in claim 1 wherein the nanoparticles comprise
Pt/C, and the electrochemical treatment of the nanoparticles comprises an
electrochemical oxidation step.

19. A method as set forth in claim 18 further comprising using the
nanoparticles in a H2/air proton exchange membrane (PEM) fuel cell
operated at high current densities to achieve higher voltage.

20. An electrochemical multi-cell comprising:a container and a liquid
electrolyte received in the container;at least two working electrodes,
and nanoparticles supported by the working electrodes;at least one
counter electrode; andat least two polymer electrolyte membranes
separating liquid electrolyte in counter electrode compartments from
liquid electrolyte in working electrode compartments in the multi-cell.

21. An electrochemical multi-cell as set forth in claim 20 wherein the
nanoparticles comprise at least one of Pt, Pt alloy, Ni, or other noble
metals or metal alloys.

22. An electrochemical multi-cell as set forth in claim 20 wherein the
working electrodes comprise a first carbon cloth supporting the
nanoparticles, and a gauze comprising a metal supporting the carbon
cloth.

23. An electrochemical multi-cell as set forth in claim 22 wherein the
gauze comprises at least one of platinum or gold or graphite.

24. An electrochemical multi-cell as set forth in claim 20 wherein the
counter electrodes comprise a second carbon cloth supported by a gauze
comprising a metal.

25. An electrochemical multi-cell as set forth in claim 24 wherein the
gauze comprises at least one of platinum or gold or graphite.

Description:

TECHNICAL FIELD

[0001]The field to which the disclosure generally relates includes methods
of treating nanoparticles.

BACKGROUND

[0002]The electrochemical treatment of large quantities of nanoparticles,
including coating, stripping, oxidation, reduction, cleaning, dealloying
of nanoparticles and so on, has long been a technical barrier for more
extensive applications of this technique in many fields such as for fuel
cells, batteries, and heterocatalysis. Heretofore, such electrochemical
treatment has resulted in non-uniform treatment of the nanoparticles.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

[0003]One embodiment of the invention includes a method of using an
electrochemical cell including a liquid electrolyte, a working electrode
with nanoparticles supported thereon, a counter electrode, and a polymer
electrolyte membrane completely separating the liquid electrolyte at the
working electrode side and liquid electrolyte at the counter electrode
side.

[0004]Other exemplary embodiments of the invention will become apparent
from the detailed description provided hereinafter. It should be
understood that the detailed description and specific examples, while
disclosing exemplary embodiments of the invention, are intended for
purposes of illustration only and are not intended to limit the scope of
the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]Exemplary embodiments of the invention will become more fully
understood from the detailed description and the accompanying drawings,
wherein:

[0006]FIG. 1 illustrates an electrode chemical cell according to one
embodiment of the invention.

[0007]FIG. 2 is a graph showing a comparison of the platinum supported on
graphitized carbon pre-oxidation curve at 1.2V(RHE) obtained by this cell
design versus by a conventional electrochemical cell.

[0008]FIG. 3 is a graph showing a comparison of fuel cell performance data
for membrane electrode assemblies (MEAs) containing the 1.4V-pretreated
Pt on graphitized carbon as cathode catalyst and for MEAs of non-treated
Pt on graphitized carbon catalyst.

[0009]FIG. 4 illustrates a multi-cell according to one embodiment of the
invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0010]The following description of the embodiment(s) is merely exemplary
in nature and is in no way intended to limit the invention, its
application, or uses.

[0011]FIG. 1 illustrates an electrochemical cell 10 according to one
embodiment of the invention. The electrochemical cell 10 may include a
container 12 that holds a liquid electrolyte 14. The liquid electrolyte
14 may be an aqueous acid solution, for example including perchloric
acid, sulfuric acid, or phosphoric acid. The liquid electrolyte may also
be any salt solution, like copper sulfate, lead sulfate, copper nitrate;
or combination of salt and acid solutions. The container 12 may be made
from any of a variety of materials, for example PTFE, glass, or other
acid resistant material. The electrochemical cell 10 may include a
working electrode 16 and a counter electrode 22. Suitable material for
the working electrode 16 and counter electrode 22 include, but are not
limited to, metals such as Pt, Au, or graphite. The working electrode 16
and the counter electrode 22 may be in the form of gauze. The gauze
material serves the function of increasing the contact area and
decreasing the mass transport resistance. The electrochemical cell 10 may
include nanoparticles 20 to be treated, which may be spread on a support
material 18 such as a first carbon cloth. The nanoparticles 20 are
electrically conductive and may be solid particle, shells with hollow
cores, or strands of connected particles. For example, the nanoparticles
20 may include, but are not limited to carbon, Pt or Pt alloy, Ni or
other metals, TiO2, or electrically conductive shells. The function
of the first carbon cloth 18 is increasing the contact area between the
nanoparticles and the supporting material. The support material or first
carbon cloth 18 is further supported by the working electrode 16 which
may be a gauze material including, for example, platinum, gold, or
graphite. A second platinum gauze and a second carbon cloth 24 are used
as the counter electrode. The counter electrode may contain a layer of
Pt/C nanoparticles or Pt black spread on the carbon cloth. The function
of these Pt/C nanoparticles is to increase the active surface area of the
counter electrode 22. Depending on the electrochemical reaction occurring
on the counter electrode 22, the material of counter electrode may also
be Cu, Pb, Ag, or other metals or metal alloys. The arrangement of the
gauze working electrode 16, overlying first carbon cloth 18, and the
gauze counter electrode 22 and underlying second carbon cloth 24
minimizes the in-cell electronic resistance which can cause non-uniform
potential distribution in the working electrode 16 and counter electrode
22. The electronic resistance in the thickness direction is very small.

[0012]A polymer electrolyte membrane 26 is interposed between the support
material 18 and the counter electrode 22 so that the polymer electrolyte
membrane serves to separate a working electrode compartment 7 and a
counter electrode compartment 9 of the cell 10 wherein the polymer
electrolyte membrane 26 completely separates the liquid electrolyte 14 in
the working electrode compartment 7 from the liquid electrolyte 14 in the
counter electrode compartment 9 of the cell 10. The second carbon cloth
24 may be interposed between the counter electrode 22 and the polymer
electrolyte membrane 26. The function of the second carbon cloth 24 is to
reduce the stress that the Pt gauze applies on the membrane. The second
carbon cloth may also function as a support for Pt/C nanoparticles, in
case a layer of Pt/C nanoparticles or Pt black is included as a part of
the counter electrode 22.

[0013]In one embodiment of the invention, the working electrode 16, first
carbon cloth 18, nanoparticles 20, membrane 26, and optionally the second
carbon cloth 24 and counter electrode 22 are all supported by the
container 12. This prevent damage to materials such as the membrane 26.

[0014]A reference electrode 28 may be provided immersed in the liquid
electrolyte 14 on the working electrode side of the cell 10. Suitable
reference electrodes 28 include, but are not limited to a Ag/AgCl
electrode, a Calomel electrode, or a reversible hydrogen electrode. A gas
purge tube 30 may be provided immersed in the liquid electrolyte 14 in
the working electrode compartment 7 of the cell 10. A cover 32 may be
placed over the container 12 with a seal or gasket 34 interposed between
the cover 32 and the container 12. Both the cover 32 and the container 12
may be made from a material including, but not limited to,
polytetrafluororethylene, glass, or other acid-resistant material. A
potential is applied across the electrodes to treat the nanoparticles 20,
using an energy source such as a battery. This arrangement may be
utilized for coating, stripping, oxidation, reduction, cleaning, or
dealloying the nanoparticles 20.

[0015]This design ensures uniform potential and uniform current density
distribution throughout the working electrode 16 and counter electrode 22
even at high current conditions and consequently ensures a uniform and
highly efficient electrochemical treatment of the nanoparticles. The cell
design combines some advantages of the polymer electrolyte membrane fuel
cell and some of the conventional liquid electrolyte electrochemical
cell. In the case where the electrochemical reaction at the counter
electrode 22 is not the reverse reaction of the working electrode 16 (for
example when H2 or O2 evolution occurs at the counter
electrode), the design can easily prevent the reaction products (H2
or O2) from diffusing into the working electrode 16. As the
nanoparticles 20 are immersed in the liquid electrolyte 14, the
utilization of the nanoparticles 20 approaches 100%, i.e. all of the
nanoparticles 20 can be treated and can be easily washed out after the
treatment. Neither of these features can be achieved for the catalyst
layer in a polymer electrolyte membrane fuel cell, in which the catalyst
layer is mixed with a solid ionomer phase.

[0016]As an example, FIG. 2 shows a comparison of the platinum supported
on graphitized carbon (Pt/GrC) pre-oxidation current at 1.2V(RHE) by
using an electrochemical cell according to the present invention versus
the same process in a conventional electrochemical cell. The much higher
current for the conventional cell is ascribed to the oxidation of H2
diffusing from the counter electrode, which is not a desirable process
and prevents monitoring the progress of the desired treatment of the
nanoparticles through a simple current measurement. The actual Pt/GrC
pre-oxidation current is achieved with the electrochemical cell according
to one embodiment of the invention, with the current dropping down to
less than 10 mA/g(Pt/GrC) in the initial 10 minutes. As such, the
electrochemical cell shown in FIG. 1 can be utilized to electrochemically
treat large quantities of nanoparticles with uniformity, high efficiency,
and facile monitoring of the state of progress of the treatment.

[0017]As an example of an application of this cell, FIG. 3 shows that
pre-oxidized Pt nanoparticles supported on graphitized carbon by using
the present invention give higher fuel cell performance than non-treated
Pt nanoparticles supported on graphitized carbon. FIG. 3 shows a
comparison of fuel cell performance data at the conditions indicated in
the graph for various membrane electrode assemblies (MEAs), which refers
to the combination of the anode catalyst, cathode catalyst, and the
membrane. The solid curves are for MEAs containing the 1.4V-pretreated Pt
on graphitized carbon as cathode catalyst. The dashed curves are for MEAs
of non-treated Pt on graphitized carbon catalyst. At 1.5 A/cm2, the
improvement is 25 mV. At 0.6 A/cm2, the improvement is as much as 50
mV. In one embodiment, the nanoparticles 20 used in a H2/air proton
exchange membrane (PEM) fuel cell operated at high current densities can
achieve higher voltage.

[0018]In various embodiments, the polymer electrolyte membrane 26 may
include a variety of different types of membranes. The polymer
electrolyte membrane 26 useful in various embodiments of the invention
may be an ion-conductive material. Examples of suitable membranes are
disclosed in U.S. Pat. Nos. 4,272,353 and 3,134,689, and in the Journal
of Power Sources, Volume 28 (1990), pages 367-387. Such membranes are
also known as ion exchange resin membranes. The resins include ionic
groups in their polymeric structure; one ionic component for which is
fixed or retained by the polymeric matrix and at least one other ionic
component being a mobile replaceable ion electrostatically associated
with the fixed component. The ability of the mobile ion to be replaced
under appropriate conditions with other ions imparts ion exchange
characteristics to these materials.

[0019]The ion exchange resins can be prepared by polymerizing a mixture of
ingredients, one of which contains an ionic constituent. One broad class
of cationic exchange, proton conductive resins is the so-called sulfonic
acid cationic exchange resin. In the sulfonic acid membranes, the
cationic exchange groups are sulfonic acid groups which are attached to
the polymer backbone.

[0020]The formation of these ion exchange resins into membranes or chutes
is well-known to those skilled in the art. The preferred type is
perfluorinated sulfonic acid polymer electrolyte in which the entire
membrane structure has ionic exchange characteristics. These membranes
are commercially available, and a typical example of a commercial
sulfonic perfluorocarbon proton conductive membrane is sold by E. I.
DuPont D Nemours & Company under the trade designation NAFION. Other such
membranes are available from Asahi Glass and Asahi Chemical Company.

[0021]The use of other types of membranes, such as, but not limited to,
perfluorinated cation-exchange membranes, hydrocarbon based
cation-exchange membranes as well as anion-exchange membranes are also
within the scope of the invention.

[0022]The electrochemical cell 10 may be used to coat nanoparticles 20
with a catalyst such as platinum to provide a plurality of supported
catalyst particles. The supported catalyst particles may be combined with
an ionomer which may be the same as the material for the above described
membrane material. The supported catalyst particles and ionomer may be
applied to both faces of a polymer electrolyte membrane of a fuel cell.
The supported catalyst particles and ionomer may alternatively be applied
to a fuel cell gas diffusion media layer or onto a decal backing for
later application as desired.

[0023]The above description is for a single cell design. Another
embodiment of the invention includes a multi-cell design or
electrochemical multi-cell 38. A schematic drawing of one embodiment is
shown in FIG. 4, wherein 40, 44, 46, 50, 52, and 56 are working
electrodes similar to the working electrode 16 described above. The
working electrodes 40, 44, 46, 50, 52, and 56 contain nanoparticles 20 to
be treated, supported on Pt or Au gauze or on other highly electronically
conductive and acid-resistant materials. These working electrodes may be
supported or sandwiched by backing material. The appropriate types of
backing materials include but are not limited to perforated PTFE board.
The multi-cell design 38 also includes counter electrodes 42, 48, and 54.
Depending on the electrochemical reaction occurring on the counter
electrode, the material of counter electrodes 42, 48, and 54 may include
Pt, Cu, Pb, Ag, or other metals or metal alloys. An electrolyte 60 fills
each of working electrode compartment 64 and counter electrode
compartment 66. Membranes 62 separate the electrolyte in the working
electrode compartments 64 from that in the counter electrode compartments
66. The multi-cell design 38 may include a container 58, which may be
glass, PTFE or other acid-resistant material. In one embodiment, the
multi-cell may have a cover made of acid resistant material (not shown).
Gas may be purged into each compartment. A reference electrode (not
shown) may be placed close to any of the working electrodes. One counter
electrode may be shared by multiple working electrodes.

[0024]When the terms "over", "overlying", "overlies" or "under",
underlying" or "underlies" or the like are used herein with respect to
the relative position of layers or components to each other such shall
mean that the layers or components are in direct contact with each other
or that another layer, layers, component or components may be interposed
between the layers components.

[0025]The above description of embodiments of the invention is merely
exemplary in nature and, thus, variations thereof are not to be regarded
as a departure from the spirit and scope of the invention.